Abstract

A soluble IL-2 receptor (sIL-2R) is a circulating form of a membrane receptor
localized on lymphoid and some cancer cells. The biological function of sIL-2R has
not been completely understood. Substantially, it seems to reflect T-lymphocyte
activation in diseases of different pathology. Moreover, the soluble receptor has been
considered, at least in part, responsible for unsuccessful immunotherapy with IL-2 in
cancers. Several lines of evidence indicate sIL-2R measurements to be useful in
determining disease progress and prognosis. This review summarizes current knowledge
on the sIL-2R behavior in RA and solid cancers of varied etiology.

INTRODUCTION

In 1984 Rubin and coworkers [1] reported the
presence of soluble IL-2 receptors in cultured human T-cell leukemia virus I (HTLV I)-positive
T lymphocytes and peripheral blood mononuclear cells (PBMCs) stimulated with mitogens.
Ever since then, a number of research surveys have been conducted to gain a better
understanding of the substance and the functions that sIL-2Rs play in the immune system.
Twenty years after the report of the Rubin's group, the biological function of sIL-2R
has not yet been completely understood, though it is regarded a marker of T-cell activation
[1]. Significantly, taking into consideration its high
(as compared to IL-2) serum concentration, sIL-2R measurements in serum/plasma give a
better tool for the assessment of the immune system activity.

STRUCTURE AND FEATURES

Soluble IL-2R is part of a membrane receptor for interleukin-2, which can be localized
on the cell surface of different lymphoid cell lines including activated T and NK cells, monocytes,
eosinophils [2, 3,
4], and on some tumor cells [5,
6, 7]. IL-2R ectodomains are
thought to be proteolytically cleaved from the cell surface [8,
9, 10] and not produced
as a result of posttranscriptional splicing [11]. This membrane receptor
is important for cell stimulation with interleukin-2 (IL-2), which is one of the most significant interleukins in the
immune system. IL-2R exists in three different forms: alpha (IL-2Rα, CD25, previously
Tac antigen, M = 55 kd), beta (IL-2Rβ, CD122, M = 75 kd), and gamma
chains (IL-2Rγ, CD132, M = 64 kd). A model of an IL-2 receptor is shown
in Figure 1.

Being shared with other cytokine receptors, beta and gamma chains belong to a cytokine receptor superfamily
[12], also called a hematopoietin receptor family
[13]. The beta subunit is common to IL-15
receptor [14], and the gamma chain
(known also as “common gamma chain,” γc)
to IL-4, IL-7, IL-9, and IL-15 receptors [14,
15, 16,
17]. Soluble IL-2Rβ can
be found in the supernatants of stimulated peripheral blood lymphocytes
[18] and during inflammatory diseases
in the serum [19],
whereas sIL-2Rγ can be found in the serum
[20] and in synovial fluid (SF)
[21], but not in the PHA-activated human
PBL cultures [22].

The IL-2R subunits demonstrate different binding affinities to IL-2,
with the highest noted for a structure composed of all the three subunits,
in which the alpha chain is required for the receptor clustering and IL-2 signal
transduction [23]. Beta and gamma subunits show intermediate
affinities, while the alpha chain alone shows the lowest and is not capable of signal
transmitting into cells [24]. This latter structure is not
normally present on the resting cells. Cellular expression of the alpha receptor followed
by its release into the circulation takes place upon lymphocyte stimulation.

Structurally, the alpha chain is not related to the cytokine receptor family.
As a membrane receptor it is a 251-amino-acid-residue polypeptide, which is
organized into two sushi (CCP/SCR) domains linked to each other with a
30-amino-acid chain [25],
required for IL-2 binding [26].
The extracellular domains are the largest part of the receptor, which consists
of 219 residues, anchored to the cell with a small 19-residual transmembrane
domain, and a cytoplasmic domain consisting of 13 amino acids
[27]. Therefore, the soluble
form that comprises the extracellular part is only about 10 kd lighter
than the membrane-bound receptor. Hence, the molecular weight of
the soluble receptor has been estimated as 35–40 kd in the case of HTLV I-positive
T cells and 45–50 kd for activated PBMC [1].

BIOLOGICAL SIGNIFICANCE

Amongst the three subunits which can be released from the cell surface,
sIL-2Rα appears to possess the best diagnostic value
in a number of diseases associated with T-cell stimulation. Substantially,
membrane receptor expression and release take place after leukocyte stimulation;
therefore, the presence of the alpha chain in the circulation is a good measure of
T-cell activation. Another advantage is its specificity to only the IL-2 receptor,
whilst the beta and gamma chains are shared with other cytokine receptors.
Moreover, contrary to gamma-chain levels, considerable amounts of the alpha
chain can be found in the serum [22].
The majority of the studies, therefore, focused on sIL-2Rα,
which can be measured not only in the serum/plasma
[28], but also in other bodily fluids,
including synovial fluid, cerebrospinal fluid [29],
and urine [28].

One of the most interesting biological features of sIL-2R is its ability to bind IL-2
with an affinity similar to that of the form present on the cell surface
[30]. Such findings are suggestive of the
immunosuppressive function attributed to this molecule. A proposed mechanism of
this interaction is presented in Figure 2. Gooding
et al [31] investigated IL-2 bioavailability during
immunotherapy with this interleukin, as influenced by the high concentrations of its
soluble receptors. They found that the elevated sIL-2R levels may lead to a decreased
cellular response to IL-2. Hence sIL-2R determination in plasma/serum may be helpful
for qualifying patients to receive IL-2 immunotherapy
[31].

Immunosuppressive function of sIL-2R. (1) An internalized antigen is presented by
MHC on APC to TCR. In response to stimulation with the antigen, activated T cells
produce IL-2 and express IL-2α receptor on their surface.
(2) IL-2 binds to...

When considering the usefulness of sIL-2R for clinical assessment, one must pay
attention to its variability caused by several intrinsic factors and keep this in mind during
the selection of control groups for clinical trials. Amongst these, one is age dependence
[28, 29,
30, 31,
32]. Gotoh et al [28] established that serum and urine
sIL-2R concentrations in childhood (age 1–14 years) appear to be 2 times higher than those in adulthood (age 21–67 years).
Similar results were obtained by Sack et al [32],
who measured serum sIL-2R in a group of 275 children
between 3 and 17 years of age. In this study sIL-2R concentration decreased along
with the age of the children, but remained higher than in the adults. High in childhood,
soluble receptor concentration rises again in adulthood, resulting in a tendency of older
people to have higher levels of this protein than young adults
[33, 34].

Food intake is also one of the factors that should be taken into consideration when
studying sIL-2R levels. Nutrition is of great importance to immunity and normal immune
responses. An adequate nutrient supply provides integrity to the immune system.
In this context, the dramatic restriction of food intake must be associated with depression
of the immune defense. This thesis had been investigated in two studies, which brought
forth contradictory results. Nagata et al [35]
reported the significant decrease of serum sIL-2R levels in anorexia. Unfortunately this
survey was performed on a very small group of anorectic subjects who were underweight
or normal. A more extensive study by Allende et al
[36], in which anorectic subjects were
divided into different groups depending on their nutritional status, did not corroborate
these findings. Although several immunological parameters were distorted in underweight patients,
the sIL-2R concentration did not differ from that of the controls. This somewhat limited
evidence, and the discrepancies between the studies, shows that more attention should
be focused on the sIL-2R concentration as influenced by the nutritional status, especially
in the cancer anorexia/cachexia syndrome.

Other factors, including time of day or night, nocturnal sleep and nocturnal
wakefulness, do not alter serum sIL-2R concentration
[37].

AUTOIMMUNE DISEASES AND CANCERS

Autoimmune diseases and cancers were found to be associated with an impairment in the T-cell-mediated immunity [38],
and IL-2 and its membrane receptor were established to be crucial to this process
[39, 40]. At present, IL-2 is not only thought of as a T-cell growth factor, but also as a factor of immune self-tolerance [41].

Autoimmunization is a process that comprises of intensified production of the proinflammatory IL-1, IL-6,
and TNF-α, and a decrease in the level of anti-inflammatory IL-2. IL-6 is known for its ability to suppress T-cell responses. In addition to this,
elevated sIL-2R levels are at least in part responsible for the depression of the IL-2-dependent immunity.

Table 1 summarizes results of the clinical trials which aimed to establish serum sIL-2R concentrations in RA and cancers.
This data provides information on the sIL-2R utility for cancer staging.

Rheumatoid arthritis

Rheumatoid arthritis (RA) is an inflammatory disease leading to joint destruction.
The molecular mechanism of synovitis is associated with T-cell activation and an elevated
production of proinflammatory cytokines, metalloproteinases, and adhesion molecules.
In human studies, an increase of sIL-2R levels during this process has been noted, both
in serum/plasma and in synovial fluid (SF) [8,
42, 43,
44]. It was established that high levels of sIL-2R
found in SF are produced by mononuclear cells [45].
Cultured PBMCs from RA subjects release considerable amounts of sIL-2R spontaneously.
No correlation, however, can be seen between the membrane receptor expression and its
release [46].

Detailed clinical trials show that serum sIL-2R levels are related to disease duration
[44] and a decline in sIL-2R concentration may
result from joint improvement [47].
Interestingly, Klimiuk et al [48] established
that serum sIL-2R concentration is related to a histological pattern of synovitis. Earlier
studies failed to establish any correlation between these two variables
[44].

Some reports indicate relationships between sIL-2R and laboratory markers
of inflammation, erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP)
[44, 45,
46, 47,
48]. These findings were not confirmed by
Fröde et al [49]. Other studies found
positive correlations between sIL-2R in serum and IL-1beta in SF
[50]
or erythropoietin in serum [51],
and negative correlations for serum haemoglobin [51]
or 1,25-dihydroxyvitamin D3 [52],
which is a known suppressor of activated T cells.

Findings from clinical trials raise a question on whether sIL-2R concentration in
serum provides a reliable immunological marker to assess disease activity in RA.
Earlier studies reported the possible advantages of sIL-2R measurements for these
purposes [53]. Tebib et al
[44] do, however, question the utility of
sIL-2R as such a marker, since it is not specific nor sensitive to measure disease activity
in an outpatient RA population. It also does not correlate with disease activity after
pharmaceutical treatments with gold salts, methotrexate, or sulfasalazine
[54, 55,
56]. Mangge et al [57], however,
point to sIL-2R determination as relevant in monitoring juvenile RA, because it
allows the ability to ascertain disease activity in cases in which common inflammatory
parameters are unaltered. Suenaga et al [58]
suggest sIL-2R measurements to be helpful for the early diagnosis of RA in patients with
joint pain, but without symptoms of bone or joint destruction.

Cancers

Cancer growth and development is associated with stimulation of the immune system,
including enhanced IL-2R expression in immune cells and its shedding into the circulation.
Numerous studies have attempted to establish connections among clinical symptoms of neoplasm,
markers of inflammation, and sIL-2R levels in body fluids. These reports documented the usefulness of sIL-2R for monitoring
anticancer therapy, in both chemotherapy and surgical treatment [59].

Malignant cells of human lymphoid tumors are thought to be the major source of serum sIL-2R. Wasik et al
[60] demonstrated in an animal study
that the sIL-2R production depended on the tumor size. Several studies proved,
however, that not only the lymphoid cancer cells express IL-2 receptors on their surface,
but also that some nonlymphoid cancer cells do, including pulmonary carcinomas and
melanoma [5, 6, 7].
Other nonlymphoid tumors, however, such as prostatic or ovarian carcinoma,
and glioblastoma multiforme, do not seem to be the source of sIL-2R
[60].

Melanoma

Melanoma cells are capable of expressing IL-2 receptors on their surface
[5], and sIL-2R has been found to correlate
with disease progression [61]. In metastatic
melanoma, sIL-2R seems to reflect tumor burden [62].
Contrary to these findings, other researchers did not see any connection between these two
variables [63]. Elevated serum sIL-2R concentration
in advanced cutaneous melanoma can provide information about worsened patient status and
chances of survival [62].

Head and neck cancers

Similarly to other neoplasms, sIL-2R levels in head and neck cancers tend to be elevated
[64, 65].
Lai et al [66] established that an increase in
serum sIL-2R concentration in nasopharyngeal carcinoma correlates with clinical staging.
In other studies, high serum concentrations at time of diagnosis were highly correlated
with shorter survival [67]. On the other hand,
low levels of the soluble receptor point to a reduced chance of metastasis development by
cancer patients within a 3-year period. Tartour et al [67] postulated that serum sIL-2R can
be employed in head and neck cancers as an independent prognostic marker of distant metastases
development and as a marker of the patient's survival.

One of the advantages attributed to sIL-2R measurement is its response to therapy.
A study by Lai et al [66] established that regular
sIL-2R serum measurement in about 90% of patients having sIL-2R levels elevated
at diagnosis provided prognostic data to estimate the immune response to radiotherapy.

In the late nineties of the last century (1990s), Chinese researchers investigated the advantages
of photodynamic therapy (PDT) in nasopharyngeal carcinoma and its effect on the serum
IL-2R and IL-2 concentrations and NK cell activity [68].
PDT procedure included laser superficial gasification of tumor lesions and the
administration of a photosensitive agent followed by photoirradiation. The post-PDT
levels of sIL-2R were found to be significantly lower, but IL-2 levels were significantly
higher than those before the therapy. Also, a markedly increase in NK
cell activity was noted as a result of therapy.

Lung cancer

Yano et al [7] discovered that tumor cells
in pulmonary adenocarcinoma are capable of expressing IL-2R and releasing it into
circulation. In another study they also found that the receptor concentrations in
adenocarcinoma and squamous-cell carcinoma were higher in the advanced than in the
early stages [69]. In the more advanced stages
of small-cell carcinoma, however, sIL-2R concentration remained unchanged.

Aleman et al [70] found high sIL-2R and
other proinflammatory cytokine concentrations as being predictive of shorter survival
among patients with advanced lung cancers, while others gave the evidence that sIL-2R
measurements can be useful for metastases detection in non-small-cell lung carcinoma
[71]. They showed that elevated presurgical
sIL-2R levels were indicative, with a sensitivity of about 90%, of intrapulmonary
metastases [71].

Some other practical roles attributed to sIL-2R measurement are its utility in prediction of early recurrences after tumor resection [72], as well as indication of shorter survival in nonoperable patients treated with chemotherapy [73].

Breast cancer

Patients with breast cancer tend to have elevated serum sIL-2R concentrations
[74, 75,
76]. No association with the disease stage,
however, has been found [74]. An increase
in serum sIL-2R levels was observed in metastatic cancers when compared to
nonmetastatic tumors [77]. Sharma et al
[75] reported the possible immunomodulatory
effect of the IL-2 soluble receptor which suppressed infiltration of blood lymphocytes into
the tumor tissue.

Sabbioni et al [78] noted that sIL-2R
concentration in the early breast cancer stages can be influenced by the type of surgery
performed. These researchers found that women, who had undergone a total mastectomy,
had lower sIL-2R concentrations than those having conserving surgery. In opposition
to surgical operations, chemotherapy does not seem to influence serum sIL-2R levels
[76].

Ovarian cancer

In advanced epithelial ovarian cancer, high serum IL-2R levels have been found to
correlate with an impairment of T-cell response
[79]. Moreover, sIL-2R in serum and ascitic
fluid tends to be higher in advanced epithelial ovarian
cancer than in the serum and peritoneal fluid of healthy women
[80]. No correlation was found, however,
between sIL-2R levels in these two bodily fluids [80].

Renal-cell carcinoma

Renal-cell carcinoma (RCC) is associated with sIL-2R elevation in plasma,
which gradually increases with the clinical stages [31]. It has been established that RCC subjects,
having an elevated sIL-2R concentration, had a shorter rate of survival than those with a lower concentration
[31]. German researchers found that the membrane-bound IL-2
receptor expression in patients who received a perioperative pretreatment with IL-2 is accompanied by
sIL-2R release [81]. This finding may explain why IL-2
immunotherapy could be unsuccessful in many cases of RCC.

Pancreatic cancer

No associations between serum IL-2R concentration and tumor grading,
or lymph node involvement, resectability, sex, and local tumor invasion in pancreatic
adenocarcinoma were observed [82].
Interestingly, Gansauge et al [82] noted a
trend toward lower sIL-2R concentration in patients with distant metastases, which is
in opposition to other studies, in which positive trends or correlations have been
described in various metastatic cancers [62,
77, 83].
These researchers also found higher sIL-2R levels in patients who are positive to anti-p53
autoantibodies. An earlier study established that the anti-p53 autoantibodies-positive
pancreatic cancer patients are significantly less metastatic than those who are anti-p53
autoantibodies negative [84].

Colorectal cancer

According to Saito et al [83] colorectal
cancer patients who are liver metastasis positive, tend to have higher serum sIL-2R
concentrations than those without metastases. Moreover, receptor concentration was
found to be independent of the presence of lymph node metastases and not associated
with the histopathological background [83].

CONCLUSIONS

Several lines of evidence indicate sIL-2R as a non-specific marker of T-cell activation
in diseases including RA and various cancers. Therefore its application for cancer staging
seems to be rather questionable. A number of surveys, however, support the utility of sIL-2R measurements in monitoring
disease progression and dynamics, and early detection of recurrent disease. Moreover,
by reflecting immune response during anticancer therapy, it offers a tool for selecting an
appropriate treatment strategy and enables evaluation of its effectiveness.